This invention relates to methods for making windows having an infrared transparent substrate and a transparent electrically conductive semiconductor layer or coating.
Transparent conductive windows through which optical energy must pass and which have good electrical conductivity have utility in a number of applications. These include resistance heated windows, electro magnetic interference (EMI) shielded windows, anti-static windows and transparent electrodes. In U.S. Pat. No. 4,710,433, entitled TRANSPARENT CONDUCTIVE WINDOWS, COATINGS, AND METHOD OF MANUFACTURE and assigned to Northrop Corporation, the assignee of the present application, transparent conductive windows, coatings and their method of manufacture are described which employ thin conductive metal layers which are sandwiched between dielectric impedance matching layers. The devices of this prior patent afford both good electrical conductivity and transmissivity in the infrared, visible and ultra violet ranges.
Other known applicable prior art devices are discussed by J. L. Vossen in an article on transparent conducting films which appeared in Physics of Thin Films, Volume 9, published in 1977 by Academic Press. In this article, the use of semiconductor oxides forming transparent conductive films is discussed. It has been found, however, that semiconductor oxide substrates exhibit the shortcomings of low carrier lifetime, resistivity which is higher than to be desired and relatively low optical transmissivity particularly in the infrared range.
Further J. Aranovich, et. al., in "High conductivity ZnSe Films", J. Appl. Phys., 49 (4), April 1978; L. C. Olsen, et. al., in "Vacuum-evaporated Conducting ZnS Films", Appl. Phys. Lett., 34 (8), April 1979; P. Petrou, et. al., in "The Use Of ZnO In Transparent Type MIS Solar Cells", Appl. Phys. Lett., Dec. 15, 1979; J. B. Webb, et. al., in "Transparent And Highly Conductive Films Of ZnO Prepared By RF Reactive Magnetron Sputtering", Appl Phys. Lett., Oct. 15, 1981 and H. Nanto, et. al., in "Electrical And Optical Properties Of Zinc Oxide Thin Films Prepared By rf Magnetron Sputtering For Transparent Electrode Applications", J. Appl. Phys., 55 (4), Feb. 15, 1984 have studied the feasibility of depositing thin conducting films of ZnS, ZnSe or ZnO for potential use in photovoltaic heterojunctions for solar cells. For these demonstrations, with one exception, the films were deposited on glass slides. For the demonstration of P. Petrou et. al. a titanium film was interspersed between the thin film and the glass slide.
The device of the present invention affords a number of advantages over that of the aforementioned prior devices and demonstrations. These include the capability of providing transmissivity over greater bandwidths; higher transmissivity for electromagnetic waves having angles of incidence away from the normal; greater mechanical durability; increased device size and greater ease and economy of fabrication.